High efficiency, high turndown furnace system

ABSTRACT

A condensing furnace assembly and method includes a primary heat exchanger having a first zone and a second zone, and a secondary heat exchanger having a first zone and a second zone. A manifold assembly includes a first set and a second set of burners. The primary and secondary heat exchanger assemblies may include a plurality of aligned tubes. The plurality of tubes may include a first zone and a plurality of the tubes include a second zone. A first air device is in communication with the first zone portions of the primary and secondary heat exchanger and the first set of burners. A second air device is in communication with the second zone of the primary and secondary heat exchangers and the second set of burners. The assembly provides for high efficiency operation (90%+ efficiency) and high turndown operation (10:1 or 10% of a maximum firing rate).

REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application No. 62/106,926 entitled “HIGH EFFICIENCY, HIGH TURNDOWN FURNACE SYSTEM” filed on Jan. 23, 2015, which is incorporated by reference herein in its entirety.

FIELD OF INVENTION

The present disclosure generally relates to a condensing furnace system which is adapted to be installed in cabinets or ducting of an air handling system. More particularly, the present disclosure relates to a gas-fired condensing furnace system with high turndown ratio.

BACKGROUND

Condensing furnaces have been used to improve heating efficiency versus conventional furnaces, for a number of years in residential heating applications. Typical conventional gas furnaces employ a burner to combust a gaseous fuel, a primary heat exchanger for transferring heat from combustion gases to the circulating air stream, a blower to circulate return air from the space to be heated over the external surfaces of the heat exchanger and through a duct system, providing warm air to the home or structure. These furnaces often include an induced draft fan to draw out and vent the flue products from the primary heat exchanger. Condensing furnaces typically employ a secondary (recuperative) heat exchange section to transfer additional heat from the combustion products after they have passed through the primary heat exchanger, thereby able to achieve efficiencies exceeding 90% versus conventional furnaces which are limited to 83%. Higher efficiency is achieved by condensing a portion of the water vapor present in the recuperative heat exchanger. Water vapor is produced as a standard by-product of the combustion process present in the flue gases, utilizing the latent heat of vaporization (972 Btu/lb. of water condensed).

Currently, high efficiency tubular style furnaces have a single induced draft fan and a single gas manifold having a plurality of burner assemblies. The primary or leading heat exchanger may be a plurality of tubes, shells, or a single cavity wherein combustion of the fuel gas occurs. Each burner assembly is associated with a tube of the heat exchanger assembly, and each are configured to combust and fire with each of the associated tubes simultaneously when the furnace is in operation. These primary heat exchangers are made of the same conventional materials as mid-efficiency furnaces (i.e., aluminized steel, 409SS, 304SS). The recuperative heat exchanger section further cools the combustion products sufficiently to condense a portion of the water vapor present. The primary heat exchanger communicates with the recuperative coil through a common flue gas collector box. The supply airflow may be initially directed over the multiple pass primary heat exchange section and then subsequently directed over the secondary heat exchange section (recuperative coil). The condensate produced may contain corrosive fluids such as hydrochloric and sulfuric acid with a pH range of 3.5 to 6.0. The recuperative heat exchanger is typically made of high grade corrosion resistant stainless steel. Preferably, the design and operation should ensure that condensation does not occur in the primary heat exchanger section to avoid premature failure or unsafe operation.

Current and proposed building efficiency standards (ASHRAE and DOE) are mandating higher combustion efficiencies particularly for “weatherized” commercial furnaces. Clean Air and Green Building initiatives are requiring increase ventilation rates necessitating higher heat inputs to temper the make-up air for ventilation. High efficiency, condensing commercial furnaces could provide improved efficiencies in these applications.

Furnaces may be classified as ‘non-weatherized” for indoor applications or as “weatherized” for outdoor applications, such as rooftop or make-up air units. Non-weatherized furnaces typically take their return air from the heated space, directing entering airflow over the recuperative (secondary) coil first, which is suitable for entering air temperatures above 40° F.

Weatherized furnaces are typically located outdoors and, therefore, may be exposed to temperatures below freezing. In make-up air applications, the heating apparatus may be exposed to different conditions; for example, 100% outdoor air (circulating air) may be directed across the heat exchanger and, therefore, may traverse the heating unit at outdoor temperatures. In extreme northern climates, entering air may be below 0° F. This can result in reduced heat exchanger surface temperatures and reduced temperatures of the combustion gases inside the heat exchanger. It is desirable, that the design of these weatherized furnaces allow the temperatures of the gases inside the leading heat exchange section tubes to remain above their dew point to avoid excess condensation in said tubes.

Currently, typical condensing furnaces used in non-weatherized applications are not capable of operating in cool temperature environments. In fact, most manufacturers of condensing furnaces typically include warnings about limiting the minimum inlet air temperature to 40° F. and locating the furnace in a space where the temperature never drops below 40° F. as such conditions could result in freezing of condensate in the condensing recuperative coil assembly and preclude their use in “weatherized” (outdoor) applications.

“Turndown” is a ratio that refers to the operational range of a furnace and may be defined as the ratio of the maximum heat output to the minimum level of heat output which the heat exchanger may operate efficiently or controllably. Typical non-weatherized condensing furnaces have been limited to a maximum turndown of approximately 4:1 in order to avoid condensation from forming in the primary heat exchanger section. Due to the wide range of entering air temperatures for “weatherized” heating installations, higher turndown would be beneficial in controlling conditions in the heated space.

The firing rate of the burners may be controlled by modulation of gas input to provide operation over the range of turndown provided. Modulating furnaces provide improved annual fuel utilization efficiencies by maintaining nearly constant temperatures in the heated space by varying heat input based on measured temperatures of the supply (outdoor) air. Applications resembling variable air volume (VAV) and zoning systems allow air pressures in the building to remain stable by varying the supply air or directing air into different zones. Higher turndown is beneficial in these applications because the heat input can be matched over a wider range of supply airflow to maintain the desired space temperatures and building pressures while operating within the furnace manufacturer's specifications.

It is desirable that weatherized modulating commercial furnaces maintain the temperatures of the gases inside the primary (leading) heat exchange section tubes above their dew point to avoid excess condensation in said tubes even at reduced firing rates during modulated operation.

Some burner systems are capable of operating at significantly reduced gas input ratings. However, in many cases the individual burners are limited to turndowns of 5:1, in order to provide proper combustion of the fuel gas. Additionally, at turndowns greater than 5:1 the resulting gas temperatures in the leading heat exchange section would be very near or below the dew point, resulting in potential for condensation in the leading heat exchange section. This could lead to premature failure of the heat exchanger from corrosive perforation of the tubes and unsafe heat exchanger operation. Maintaining gas temperatures in the leading heat exchange section well above the dew point may assist with extending the life and safe operation of the furnace. An assembly and method can provide a high efficiency furnace with modulated operation and high turndown ratio for “weatherized” applications.

SUMMARY

The present disclosure provides a furnace assembly that can provide high efficiency. The present furnace assembly can also provide a high turndown ratio. In one aspect, this disclosure relates to a dual zone condensing furnace assembly that includes a primary heat exchanger having a first zone portion and a second zone portion, and a secondary heat exchanger having a first zone portion and a second zone portion. The condensing furnace may further include a burner manifold assembly having a first set of burners and a second set of burners. The primary heat exchanger assembly may include a plurality of aligned multiple pass tubular heat exchanger tubes having a first straight, a first turn, a second straight, a second turn, and a third straight. A plurality of the tubes make up the first zone portion, and a plurality of the tubes make up the second zone portion. The tubes may each include an input in direct communication with the burner manifold assembly and an output in communication with a first coupler box.

The secondary heat exchanger assembly may include a plurality of aligned single-pass tubular heat exchange tubes made from a corrosion-resistant material. The secondary heat exchanger assembly may include a plurality of fins. A plurality of the tubes make up the first zone portion and a plurality of the tubes make up the second zone portion. Each of the tubes may include an inlet in direct communication with the first coupler box and an outlet in communication with a second coupler box. The primary heat exchanger assembly may be adjacent to the secondary heat exchanger assembly.

The first coupler box and the second coupler box may each define cavities that include a dual zone divider that separates the cavities into the first zone portion and the second zone portion. The first zone portions of the cavities communicate with the first zone portion of the tubes of the primary and secondary heat exchanger assemblies. The second zone portions of the cavities communicate with the second zone portion of the tubes of the primary and secondary heat exchanger assemblies.

A first combustion air device is attached to the second coupler box and is in communication with the first zone portions of the first coupler box, primary heat exchanger tubes, second coupler box, and the first set of burners. A second combustion air device is in communication with the second zone portions of the first coupler box, primary heat exchanger assembly, second coupler box, second heat exchanger assembly, and the second set of burners.

The condensing furnace assembly is configured such that airflow may traverse over the first straight portion of the primary heat exchanger tubes through the second straight portion and third straight portion. The airflow then passes over the secondary heat exchanger assembly. The first set of burners and the second set of burners can be modulated to combust the first zone portions or the second zone portions such that a risk of condensation freezing within the primary and secondary heat exchange assemblies is reduced. The condensing furnace assembly provides for high efficiency operation (90%+ efficiency) and high turndown operation (10:1 or 10% or higher of maximum firing rate).

In one embodiment, provided is a dual zone condensing furnace system. The system includes a primary heat exchanger assembly having a plurality of aligned multiple pass tubular heat exchanger tubes having a first straight, a first turn, a second straight, a second turn, and a third straight wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion. A secondary heat exchanger assembly is provided in communication with the primary heat exchanger assembly and includes a plurality of aligned single-pass tubular heat exchange tubes made from a corrosion-resistant material wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion. Combustion gases may be introduced into the heat exchangers such that the combustion gases within the first zone portions do not communicate with the second zone portions and combustion gases within the second zone portions do not communicate with the first zone portions. The condensing furnace system may be operated to provide for high efficiency operation (90%+ efficiency) and high turndown operation (10:1 or 10% or higher of maximum firing rate). The airflow traversing over the heat exchanger assemblies may include an inlet temperature that is less than 32° F. and more particularly that is less than 0° F.

The dual zone condensing furnace system may include a burner manifold assembly having a first set of burners and a second set of burners wherein the first set of burners and the second set of burners can be modulated to combust the first zone portions or the second zone portions such that a risk of condensation freezing within the primary and secondary heat exchange assemblies may be reduced.

Also provided is a method for operating a dual zone condensing furnace system. The method includes providing a primary heat exchanger assembly having a plurality of aligned multiple pass tubular heat exchanger tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion. A secondary heat exchanger assembly is provided and includes a plurality of aligned tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion. An airflow to be heated may traverse over the primary and secondary heat exchanger assemblies. A burner manifold assembly may be modulated to provide combustion gas into at least one of the first zone portion and the second zone portion of the primary heat exchanger assembly such that the condensing furnace system operates at high efficiency (90%+ efficiency) and high turndown (10:1 or 10% or higher of maximum firing rate). In one embodiment, the burner manifold assembly may be modulated to provide combustion gas into only one of the first zone portion and the second zone portion of the primary heat exchanger assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

Operation of the disclosure may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:

FIG. 1 is a rear perspective view of an embodiment of a condensing furnace of the present disclosure.

FIG. 2 is a front perspective view of the condensing furnace of the present disclosure.

FIG. 3 is a partially exploded front perspective view of the condensing furnace of the present disclosure.

FIG. 4 is a top view of the condensing furnace of the present disclosure.

FIG. 5 is a rear view with a partial cutaway view of the condensing furnace of the present disclosure.

FIG. 6 is a side view of the condensing furnace of the present disclosure.

FIG. 7 is a front view with a partial cutaway view of the condensing furnace of the present disclosure.

FIG. 8 is a front view of the condensing furnace of the present disclosure.

FIG. 9 is a top view of a housing for the condensing furnace of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the respective scope of the disclosure. Moreover, features of the various embodiments may be combined or altered without departing from the scope of the disclosure. As such, the following description is presented by way of illustration only and should not limit in any way as various alternatives and modifications may be made to the illustrated embodiments and still be within the spirit and scope of the disclosure.

As illustrated by FIGS. 1-7, a dual zone condensing furnace 10 is disclosed that may be arranged within an enclosure that is in communication with a supply airflow 110 through a ventilation system (not shown). The ventilation system generally includes ductwork that handles the airflow of an HVAC system of a building.

FIGS. 1 and 4 illustrate the dual zone condensing furnace 10, which includes a primary heat exchanger assembly 20 and a secondary heat exchanger assembly 40. The primary heat exchanger assembly 20 may include a plurality of aligned multiple pass tubular heat exchanger tubes 22 having a first straight 24, a first turn 26, a second straight 28, a second turn 30, and a third straight 32. The primary heat exchanger assembly having ten tubes 22 a, 22 b, 22 c, 22 d, 22 e, 22 f, 22 g, 22 h, 22 i, and 22 j that are separated between a first zone portion 34 and a second zone portion 44. The first zone portion 34 includes a plurality of the tubes 22 a-22 e. The second zone portion 44 includes the remaining plurality of the tubes 22 f-22 j. The tubes 22 a-22 j may each include an inlet 36 attached to a tube sheet 60 in direct communication with a burner manifold assembly 46 and an outlet 38 in communication with a first coupler box 48. However, this disclosure is not limited as to the amount of primary tubes 22 or burners 80. It should be understood that the number of primary heat exchanger tubes 22 and corresponding burners 80 may be established by the required heating capacity of the furnace 10.

A part of the injection process includes drawing air into the primary heat exchanger assembly 20 so that the fuel gas and air mixture may be combusted therein. The embodiments illustrated by the Figures include ten (10) tubes 22 along with ten (10) burners 80. In this particular flow path configuration, the inlets 36 of the primary heat exchanger assembly 20 and outlets 70 of the secondary heat exchanger assembly 40 may be in communication through the tube sheet 60. Additionally, these inlets 38 and outlets 70 may be generally aligned along a common plane along the tube sheet 60. The burner assembly 48 may be mounted to an opposite side of the tube sheet 60

The plurality of first straight portions 24 attach to the plurality of first turn portions 26 and may be supported by a spacer member 94. The spacer member 94 may be configured to align and space each of the tubes 22 in a desired orientation.

The first coupler box 48 includes a dual zone divider 50 that may separate the first coupler box 48 into a first zone cavity 52 and a second zone cavity 54. The first zone cavity 52 communicates with the first zone portion 34 of the tubes of the primary heat exchanger assembly 20. The second zone cavity 54 communicates with the second zone portion 44 of the tubes 22 of the primary heat exchanger assembly 20. The dual zone divider 50 may be a baffle plate that isolates communication between the first zone cavity 52 and the second zone cavity 54.

With reference to FIGS. 2, 3, 6, 7, and 8, the secondary heat exchanger 40 may be an intermediate single-pass tubular heat exchange section that is made from a corrosion-resistant heat exchange element. The secondary heat exchanger assembly 40 may include a plurality of aligned single-pass tubular heat exchange tubes 62 made from a corrosion-resistant material. The secondary heat exchanger assembly may include a plurality of fins. The shape, size, and spacing of the fins are not particularly limited and may be selected as desired. A plurality of the tubes 62 make up a first zone portion 64, and a plurality of the tubes 62 make up a second zone portion 66. Each of the tubes 62 may include an inlet 68 in direct communication with the first coupler box 48 and an outlet 70 that may be attached to the tube sheet 60 and in communication with a second coupler box 72. The tubes 62 of the first zone portion 64 are in communication with the first zone cavity 52 of the first coupler box 48, and the tubes 62 of the second zone portion 66 are in communication with the second zone cavity 54 of the first coupler box 48. The tubes 22 of the primary heat exchanger assembly 20 may be adjacent to and in general airflow alignment to the tubes 62 of the secondary heat exchanger assembly 40, wherein the third straight portion 32 may be directly adjacent to and may be generally parallel with the secondary heat exchanger tubes 62.

The burner manifold assembly 46 may be attached to the tube sheet 60 opposite from the primary heat exchanger assembly 20 and may include a plurality of burners 80 a, 80 b, 80 c, 80 d, 80 e, 80 f, 80 g, 80 h, 80 i, and 80 j that are in communication with the plurality of tubes 22 a-22 j, respectively. The plurality of burners 80 a-80 e may be a first set of burners 82, and the remaining plurality of burners 80 f-80 j may be a second set of burners 84. The first set of burners 82 may be in communication with the first zone portion 34 of the primary heat exchanger assembly 20, and the second set of burners 84 may be in communication with the second zone portion 44 of the primary heat exchanger assembly 20. However, this disclosure is not limited as to the amount of primary tubes 22, secondary tubes 62, or burners 80. It should be understood that the number of primary heat exchanger tubes 22 and corresponding secondary heat exchanger tubes 62 and burners 80 may be established by the required heating capacity of the furnace. A part of the injection process includes drawing air into primary heat exchanger assembly 20 so that the fuel gas and air mixture may be combusted therein. This occurs when the first set of burners 82 fires into the plurality of tubes 22 a-22 e associated with the first zone portion 34 and/or when the second set of burners 84 fires into the plurality of tubes 22 f-22 j associated with the second zone portion 36.

A first combustion air device 86 and a second combustion air device 88 may be attached to the second coupling box 72. The combustion air devices 86 and 88 may be configured to draw combustion gases through the primary and secondary heat exchanger assemblies 20 and 40. In one embodiment, the combustion air devices 86 and 88 may be induced draft motor assemblies that include a motor with an inducer wheel for drawing the combustion gases. The second coupler box 72 may include a dual zone divider 90 that divides the second coupler box 72 into a first zone cavity 92 and a second zone cavity 94. The dual zone divider 90 may be a baffle plate that isolates communication between the first zone cavity 92 and the second zone cavity 94.

The first combustion air device 86 is in communication with the the first zone cavity 92 to communicate with the first zone portions 34 and 64 of the tubes 22 and 62 of the primary and secondary heat exchanger assemblies 20 and 40, respectively. The second combustion air device 88 is in communication with the second zone cavity 94 to communicate with the second zone portions 44 and 66 of the tubes 22 and 62 of the primary and secondary heat exchanger assemblies 20, 40, respectively. In one embodiment, the flue gases do not communicate between the first and second zone portions of the primary and secondary heat exchanger assemblies. The flue or combustion gases may not be transferred from one zone portion to the other.

The combustion gas enters the inlets 36 (FIG. 3) and exits the outlets 38 (FIG. 5) and flows into a first coupling box 48. The combustion gas then enters the inlets 68 (FIG. 5) of the secondary heat exchanger assembly 40 and exits the outlets 70 (FIG. 3) and flows into the second coupling box 72. The combustion gas may then be exhausted through a flue duct and condensate drain (not shown). Notably, at least a portion of this configuration may be contained within a housing 100 configured to allow the heat exchangers 20 and 40 to be in communication with the supply airflow 110 as it traverses the arrangement of heat exchangers.

The furnace assembly 10 is configured such that the supply airflow 110 traverses over the first straight portion 24 of the primary heat exchanger tubes 22 through the second straight portion 28, and third straight portion 32. The airflow then passes over the tubes 66 of the secondary heat exchanger assembly 40. The supply airflow traverses over the heat exchangers as combustion gases travel therein. The first set of burners 82 and the second set of burners 84 of the burner manifold 46 may be modulated to fire combustion gases within at least one of the first zone portion 34 and the second zone portion 44 of the primary heat exchanger assembly 20 such that a risk of condensation freezing within the primary and secondary heat exchange assemblies is reduced.

In operation, the primary heat exchanger assembly 20 receives the highest temperature combustion flue gases from the burners. The combustion flue gases remain above the dew point temperature even as heat is transferred to the air and traverses over the heat exchanger tubes. The primary heat exchanger 20 may be made of an aluminized coated steel tube system. The secondary heat exchanger 40 may include tubes having smaller sized diameters or cross sectional areas than those of the primary heat exchanger 20 to receive the exhaust gases once they have gone through the primary heat exchanger 20. Here, more heat is extracted from the combustion gases, and as a result the gases may be cooled to the point that they condense into water, carbon dioxide, and other chemical exhaust materials. These exhaust materials may form an acidic condensate such as hydrochloric and sulfuric acid. The secondary heat exchanger 40 may be made of proprietary stainless steel alloys such as super ferritic stainless steel such as AL 29-4C® provided by ATI Properties, Inc. Allegheny Technologies Inc. (ATI).

The condensing furnace 10 may include a two stage or dual stage burner assembly 46 with electronic controls that allow the burner flame to be modulated between a high and a low setting depending on the level of heat required. Additionally, the burner assembly 46 may have a modulating or variable capacity gas valve having an electronic control system for the burner and circulating air devices 86 and 88 that allow very fine adjustments to the burner setting and blower motor speed, modulating them to keep the temperature of the heated space very close to a thermostat setting or maintain a desired supply air temperature for ventilation air provided to the space.

In this configuration, the condensing furnace 10 extracts useful heat even after the combustion gases have “cooled” through the primary heat exchanger assembly 20. This is accomplished by the secondary heat exchanger assembly 40 wherein the water vapor contained in the combustion gases entering the secondary heat exchanger 40 is condensed as heat is extracted from the gases in this section.

The acid condensate resulting from the gases going through the secondary heat exchanger 40 may be drained and may be discharged through a drain pipe such as a plastic PVC pipe. The condensate may attack and corrode the furnace body or any other metal with which it comes in contact. Additionally, the condensing furnace flue exhaust gases may be relatively cool and can be vented from the combustion air devices 86 and 88 with a plastic vent pipe such as an ABS or CPVC pipe because of their low temperature of around 120° F. or less.

This condensing furnace assembly 10 may maintain internal thermal fluid and tube surface temperatures above the dew point of the exhaust materials in the primary heater tube section. Corrosion-resistant tubing materials may be utilized in the secondary heat exchanger assembly, where condensing of water vapor in flue gases occurs. The secondary heat exchanger assembly 40 may also include finned tubes to provide additional heat transfer surface to external circulating fluid and further reduce the temperatures of the internal flue gases.

The condensate produced by the combustion of gaseous fuels (i.e., natural gas, propane gas, etc.), however, is acidic and corrosive (3.5-6.0 pH) even to most stainless steel materials. In the disclosed design, the secondary and tertiary heat exchangers may be made from materials which resist corrosive attack from this condensate.

The present condensing furnace assembly provides for high efficiency operation (90%+ efficiency) and high turndown operation (10:1 or 10% or higher of maximum firing rate) having a continuous gas input modulation throughout its operating range. The output of the respective burners in the burner assembly may be turned or configured to control the turndown ratio. In the present assembly, a turndown of 10:1 (10%) may be achieved while operating the first set of burners 82 or the second set of the burners 84. In one embodiment, the burners 80 may be fixed at 20% input (10% of total input) such that combustion gas temperatures in the primary heat exchanger assembly 20 may be elevated above the dew point temperature.

In operation, for example, wherein the first set of burners 82 and the first combustion air device 86 are operated, the combustion gases flow only through the first zoned portion and first zone cavities of the primary and secondary heat exchanger assemblies 20 and 40. This may prevent dilution from unheated air and may maintain a higher flue gas temperature in the primary heat exchange assembly 20. Correspondingly, since no combustion gases are present in the second zone portion of the primary and secondary heat exchanger assemblies 20 and 40, no condensation occurs in the unfired portions at a maximum turndown condition. This configuration reduces the potential of condensate freezing in any part of the heat exchangers.

Further, during modulation of combustion gas inputs of 10:1 turndown or higher, combustion gases may be allowed to have a lower temperature than the dew point temperature in areas of the heat exchanger protected by super ferritic material.

Additionally, during conditions wherein low temperature ambient air is circulated over the heat exchangers, the operating first zone portion of the secondary heat exchanger assembly may maintain higher condensate temperatures, operating temperatures, and combustion gas temperatures thereby reducing the potential for condensate freezing in the secondary heat exchanger assembly and condensation occurring in unprotected parts of the heat exchangers.

Although the embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it is to be understood that the present invention is not to be limited to just the embodiments disclosed. The invention described herein is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the claims hereafter. The features of each embodiment described and shown herein may be combined with the features of the other embodiments described herein. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof. 

Having thus described the invention, we claim:
 1. A dual zone condensing furnace assembly comprising: a primary heat exchanger assembly having a plurality of tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion, and a secondary heat exchanger assembly having a plurality of tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion.
 2. The dual zone condensing furnace assembly of claim 1, further comprising a burner manifold assembly having a first set of burners and a second set of burners.
 3. The dual zone condensing furnace assembly of claim 1, wherein the plurality of tubes of the primary heat exchanger assembly are a plurality of aligned multiple pass tubular heat exchanger tubes having a first straight, a first turn, a second straight, a second turn, and a third straight.
 4. The dual zone condensing furnace assembly of claim 1, wherein the tubes of the primary heat exchanger assembly each include an input in communication with a burner manifold assembly and an output in communication with a first coupler box.
 5. The dual zone condensing furnace assembly of claim 1, wherein the plurality of tubes of the secondary heat exchanger assembly are a plurality of aligned single-pass tubular heat exchange tubes made from a corrosion-resistant material.
 6. The dual zone condensing furnace assembly of claim 5, wherein the secondary heat exchanger assembly includes a plurality of fins.
 7. The dual zone condensing furnace assembly of claim 5, wherein each of the tubes of the secondary heat exchanger assembly includes an inlet in communication with a first coupler box and an outlet in communication with a second coupler box.
 8. The dual zone condensing furnace assembly of claim 1, wherein combustion gases within the first zone portions do not communicate with the second zone portions and combustion gases within the second zone portions do not communicate with the first zone portions.
 9. The dual zone condensing furnace assembly of claim 1, wherein a first coupler box and a second coupler box are in communication with the primary and secondary heat exchanger assemblies wherein each box includes a dual zone divider that separates the boxes into a first zone cavity and a second zone cavity.
 10. The dual zone condensing furnace assembly of claim 9, wherein the first zone cavities communicate with the first zone portion of the tubes of the primary and secondary heat exchanger assemblies and the second zone cavities communicate with the second zone portions of the tubes of the primary and secondary heat exchanger assemblies.
 11. The dual zone condensing furnace assembly of claim 2, further comprising a first combustion air device in communication with the first zone portions of the primary and secondary heat exchanger tubes and the first set of burners.
 12. The dual zone condensing furnace assembly of claim 11, further comprising a second combustion air device in communication with the second zone portions of the primary and secondary heat exchanger assemblies and the second set of burners.
 13. The dual zone condensing furnace assembly of claim 3, wherein the condensing furnace assembly is configured such that airflow traverses over the first straight of the primary heat exchanger tubes then traverses over the second straight, and third straight before traversing over the secondary heat exchanger assembly.
 14. The dual zone condensing furnace assembly of claim 2, wherein the first set of burners and the second set of burners can be modulated to provide combustion gas to the first zone portions or the second zone portions such that a risk of condensation freezing within the primary and secondary heat exchange assemblies is reduced.
 15. The dual zone condensing furnace assembly of claim 1, wherein the condensing furnace assembly exhibits an efficiency of 90% or greater, a turndown operation of 10:1, or a combination thereof.
 16. A dual zone condensing furnace system comprising: a primary heat exchanger assembly having a plurality of aligned multiple pass tubular heat exchanger tubes having a first straight, a first turn, a second straight, a second turn, and a third straight wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion, a secondary heat exchanger assembly having a plurality of aligned single-pass tubular heat exchange tubes made from a corrosion-resistant material wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion, wherein combustion gases within the first zone portions do not communicate with the second zone portions and combustion gases within the second zone portions do not communicate with the first zone portions, and wherein the condensing furnace assembly exhibits an efficiency of 90% or greater, a turndown operation of 10:1, or a combination thereof.
 17. The dual zone condensing furnace system of claim 16, further comprising a burner manifold assembly having a first set of burners and a second set of burners wherein the first set of burners and the second set of burners can be modulated to combust the first zone portions or the second zone portions such that a risk of condensation freezing within the primary and secondary heat exchange assemblies is reduced.
 18. The dual zone condensing furnace system of claim 16 further comprising: a first coupler box and a second coupler box in communication with the primary and secondary heat exchanger assemblies, wherein each box includes a dual zone divider that separates the boxes into a first zone cavity and a second zone cavity wherein, the first zone cavities communicate with the first zone portion of the tubes of the primary and secondary heat exchanger assemblies, and the second zone cavities communicate with the second zone portions of the tubes of the primary and secondary heat exchanger assemblies.
 19. A method for operating a dual zone condensing furnace system, the steps comprising: providing a primary heat exchanger assembly having a plurality of aligned multiple pass tubular heat exchanger tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion; providing a secondary heat exchanger assembly having a plurality of aligned tubes wherein a portion of the plurality of tubes make up a first zone portion and a portion of the plurality of tubes make up a second zone portion; traversing an airflow over the primary and secondary heat exchanger assemblies; and modulating a burner manifold assembly to provide combustion gas into at least one of the first zone portion and the second zone portion of the primary heat exchanger assembly such that the condensing furnace system exhibits an efficiency of 90% or greater, a turndown operation of 10:1, or a combination thereof.
 20. A method for operating a dual zone condensing furnace system of claim 19 further comprises modulating the burner manifold assembly to provide combustion gas into only one of the first zone portion and the second zone portion of the primary heat exchanger assembly. 